Field of the Invention
the disclosure relates to bandpass filters for use in light emitting devices, and more specifically, to a bandpass filter for use in a light emitting device including a light emitting element and a phosphor-containing layer containing a phosphor for emitting light by being excited by light from the light emitting element, and a light emitting device using the bandpass filter.
Description of Related Art
Light emitting devices using a light emitting element, such as a light emitting diode (LED) or a laser diode (LD), are generally known to be small in size with a good light emitting efficiency (power efficiency) and to emit vivid-colored light.
A spectrum of light emitted from such a light emitting element generally has its peak in the narrow wavelength range of the spectrum. In contrast, for example, light emitting devices used for applications emphasizing visibility of color, including an illumination equipment, a display, and the like are required to emit light with high color rendering properties, for example, to emit light having a relatively wide spectrum, such as white light.
Thus, most of the light emitting devices with the light emitting element and used for these applications include a phosphor-containing layer containing one or more kinds of phosphors selected from, for example, a green phosphor, a yellow phosphor and a red phosphor. Then, the phosphor absorbs the light emitted from the light emitting element, and converts the absorbed light into another light with a different wavelength (in a different color). As a result, the light emitted from the light emitting element and passing through the phosphor-containing layer without being absorbed in the phosphor is mixed with the light converted by the phosphor and discharged from the phosphor-containing layer, thereby producing light with a wide range of spectrum.
However, the provision of the phosphor-containing layer allows a part of the light emitted from the light emitting element to reflect from the surface of the phosphor-containing layer without entering the phosphor-containing layer, so that the part of the light does not contribute to the brightness of the light emitted from the light emitting device toward the outside. This leads to the reduction in efficiency of the light emitting device.
Further, a part of light emitted by the phosphor is discharged from one surface of the phosphor-containing layer opposite to the light emitting surface thereof, that is, from the light emitting element side surface, so that the part of the light does not also contribute to the brightness of the light emitted from the light emitting device toward the outside. This also leads to the reduction in efficiency of the light emitting device.
In order to prevent the reduction in efficiency of the light emitting device, a method is proposed which involves forming a bandpass filter on a surface of the phosphor-containing layer on a light emitting element side (or a surface opposite to the light emitting element) to allow the light in a specific wavelength range to preferentially pass through the phosphor-containing layer, as disclosed in JP 2010-108965 A.
The bandpass filter is a multilayer film composed of at least two kinds of dielectric layers. The bandpass filter has a high transmittance (that is, a low reflectivity) for the light emitted from the light emitting element, and a low transmittance (that is, a high reflectivity) for the light emitted from the phosphor excited by the light from the light emitting element.
The amount of light reflected from the surface of the bandpass filter of the light emitted from the light emitting element is smaller than that of light reflected from the surface of the phosphor-containing layer without using the bandpass filter. Most of the light entering into the bandpass filter can enter the phosphor-containing layer as it is. Thus, most of the light emitted from the light emitting element can be guided to the phosphor-containing layer.
Further, most of light emitted from the surface on the light emitting element side of the phosphor-containing layer in the lights excited by the phosphor is reflected from the surface of the bandpass filter to return to the inside of the phosphor-containing layer.
As a result, the light emitting device with the bandpass filter can obtain the high light emission efficiency.
In many applications using the light emitting device, including an illuminating device, a display device, or the like, the reduction in used energy (energy saving) and in size and weight of the device are requested more than ever before. Also, the light emitting device is also required to have further improved light emission efficiency.